1. Field of the Invention
The present invention relates to disk drive devices and particularly to disk drive devices that reduce evaporation of lubricant in fluid dynamic bearings.
2. Description of the Related Art
There are disk drive devices, such as Hard Disk Drives, in which fluid dynamic bearing units are mounted that allow for stable high-speed rotation. For example, lubricant filling between a sleeve forming part of a stator and a shaft forming a part of a rotor in a motor provided with the dynamic pressure bearing described in patent document 1. A dynamic pressure produced in the lubricant supports the rotor in a non-contact state, thereby smooth and high-speed rotation is realized.
[Patent document 1] Japanese Laid-Open Publication No. 2008-275047
A disk drive device, on which a fluid dynamic bearing is mounted, that can be used for a longer term is required. There is a problem where the lubricant filling the fluid dynamic bearing is depleted when the disk drive device is used for a long term. When the lubricant is depleted and results in a shortage, the fluid dynamic bearing cannot generate a sufficient dynamic pressure, and the accuracy of the rotation may be decreased. In the worst case, a malfunction, such as burn-in, of the fluid dynamic bearing may be caused. A major factor for such a depletion of the lubricant is a phenomenon where the amount of lubricant is reduced due to evaporation.
With regard to the evaporation of the lubricant filling in the fluid dynamic bearing, when the vapor-liquid interface of the lubricant is in contact with the outside air, the evaporated lubricant gradually dissipates into the outside air, causing a problem where the lubricant becomes short when the cumulative amount of the evaporation exceeds a pre-set acceptable amount. Therefore, by increasing the time required for the cumulative amount of the evaporation to exceed the pre-set acceptable amount (hereinafter, referred to as “evaporation threshold time”), the lifetime of the disk drive device can be increased. However, increasing the volume of the lubricant so as to increase the evaporation threshold time can result in an increase in the size of the disk drive device.